Significant investment has been made in developing optical methods for the remote or stand-off detection of trace chemicals, environmental pollutants, high explosives, and chemical and biological agents. In particular, detection can be achieved by laser remote sensing based on Raman scattering or laser-induced fluorescence from a target sample. Therefore, radiation detected at wavelengths different from that of the laser's output can contain highly specific molecular information that can be used to determine the composition of the target sample.
Raman spectroscopy uses a single frequency of radiation to irradiate a sample and detects the inelastically scattered radiation that is one vibrational unit of energy different from the incident radiation. Raman scattering is strongest when vibrations cause a change in the polarizability of the electron cloud around the molecule. Therefore, the difference in energy between the incident and scattered photons is a characteristic of and provides structural information about the irradiated molecule. Further, due to its narrow spectral lines and unique signatures, Raman spectroscopy enables selective identification of individual analytes in a complex, multicomponent mixture without the need for chemical separations. In addition, the technique requires little or no sample preparation, is nondestructive, and can use water as a solvent (since water is a poor Raman scatterer). The intensity of the scattering is related to the power of the laser used to excite the scattering, the square of the polarizability of the molecule, and the fourth power of the frequency of the exciting laser. Therefore, the most common choice is a visible laser for excitation.
Unfortunately, Raman scattering is an inherently weak process, precluding the possibility of remote trace analysis without some form of enhancement. However, surface-enhanced Raman scattering (SERS) can give an enhancement of up to about 106-107 in scattering efficiency over normal Raman scattering. Even stronger enhancements, of order 1011-1013, come from sharp features or “hot spots”, such as are found in nanostructures. Such extremely large enhancements can produce a total SERS cross-section comparable to that of fluorescence.
In particular, SERS can give molecularly specific information about an adsorbate on a roughened metal surface and can be carried out in a wide range of environments. When the metal surface is irradiated by the incident laser light, conduction electrons in the metal are displaced into an oscillation of frequency equal to the incident light. When spatially confined, for example by a roughened surface, these oscillating electrons, or surface plasmons, produce a secondary electric field that adds to the incident field. The interaction between the sample and the plasmons can occur by either electromagnetic or chemical enhancement. With electromagnetic enhancement, the excitation of the surface plasmon greatly increases the local field of the molecule absorbed on the surface, increasing the polarization around the molecule. Although electromagnetic enhancement does not require direct contact of the molecule with the metal, the dependence on distance is extremely strong. Chemical enhancement involves the formation of a bond between the molecule and the metal surface, enabling charge transfer from the metal surface to the molecule, again increasing the molecular polarizability. Enhancement is maximized when both the incident laser and Raman scattered fields are in resonance with the surface plasmons. Such highly localized surface plasmons are thought to produce very strong fields, or “hot spots”, over areas as small as a few nanometers, enabling single-molecule detection. Silver is a particularly good substrate for SERS, although other metals, such as gold and copper, also give good enhancement. Both silver and gold plasmons oscillate at frequencies in the visible region, suitable for use with a visible laser. If the surface metal film is thin (e.g., less than 15 nm), the much larger surface plasmon spans the film and is operative on both sides. Thus, the interaction between the molecule and the plasmon on one side of the film can be detected on the other side. See F. Yan et al., “Surface-Enhanced Raman Scattering Detection of Chemical and Biological Agent Simulants,” IEEE Sensors Journal 5(4), 665 (2005), which is incorporated by reference.
With laser-induced fluorescence, the laser radiation is matched to a specific electronic transition of the atom or molecule, or fluorophore, which subsequently emits radiation at a lower frequency (i.e., longer wavelength). Typically, a tunable ultraviolet laser source can be used to excite visible fluorescence. Although typically much more efficient than Raman scattering, broadband emission is observed with most molecules. Therefore, laser-induced fluorescence provides less specific molecular information than Raman scattering. However, multispectral analysis algorithms can be used with a database of fluorescence signatures to obtain species concentrations. See P. J. Hargis et al., “Multispectral ultraviolet fluorescence lidar for environmental monitoring,” Proc. of SPIE 2366, 394 (1995); and R. J. Simonson et al., “Remote Detection of Nitroaromatic Explosives in Soil using Distributed Sensor Particles,” Proc. of SPIE 4394, 879 (2001); which are incorporated herein by reference.
Surface-enhanced fluorescence (SEF), or metal-enhanced fluorescence, has also been observed for weakly fluorescent substances placed at suitable distances (e.g., 5-20 nm) from metallic surfaces and particles (e.g., metal colloids or islands). Depending upon the distance and geometry, metal surfaces or particles can result in enhancement of fluorescence by factors of 103. This enhancement results from the fluorophore dipole interacting with free electrons in the metal. Proximity to nearby metallic surfaces can also increase the local light field and modify the rate of excitation. See C. D. Geddes and J. R. Lakowicz, “Metal-Enhanced Fluorescence,” J. Fluor. 12(2), 121 (2002), and J. R. Lakowicz, “Radiative Decay Engineering: Biophysical and Biomedical Applications,” Anal. Biochem. 298, 1 (2001), which are incorporated herein by reference.
A Light Detection and Ranging (LIDAR) instrument can be used to obtain remote SERS or fluorescence measurement of samples. A LIDAR instrument comprises a laser source for irradiation of the remote sample, a collection telescope for collecting the returned signal from the sample, and a spectrally-resolved photodetector to detect the returned signal. See R. M. Measures, Laser Remote Sensing: Fundamentals and Applications, Wiley-Interscience (New York) 1984.
However, in the usual LIDAR system, the backscattered light is nondirectional and the returned signal falls off by an inverse-square dependence with range. Therefore, a need remains for laser remote sensing apparatus wherein the backscattered light is returned to the detector with high efficiency to enable trace analysis of remote chemical or biological samples.